Millennium was a mid 90's partnership between John Houck and Harold Duvall. Basically nobody thought cheap disc golfers would pay extra for better plastic, but John and Harold did and started Millennium. When all other discs were made of base plastic and sold for $7, Millennium discs were made from the equivalent of today's Pro plastic and sold for $10. It worked, and pretty soon Discraft and Innova had their own premium plastic lines.
Harold Duvall is one of the Innova partners, so the idea was never for Millennium to mold their own discs. The original discs were the Omega (made from the small bead Aviar mold) the Aurora MS (Shark top on a Stingray bevel) and the Tachyon LS & LF (made from Raven and Viper mold pieces.) All of their discs have been made by Innova and shared various mold pieces with other Innova discs.
A few years back Harold Duvall left the company, so now it is 100% Houck's company. Innova still makes all of their discs.